In a commercial video over DSL broadcast system, it is desirable to allow end users to be able to change channels rapidly. Popular video compression standards, such as MPEG-2 and JVT/H.264/MPEG AVC use intra and inter coding. For proper decoding, a decoder must decode a compressed video sequence beginning with an intra-coded (I) picture, and then continue to decode the subsequent inter-coded (P and B) pictures. A Group of Pictures (GOP) may include an I picture and several subsequent P and B pictures. I pictures typically require many more bits to code than does a P or B picture of equivalent video quality, in the range of 3 to 10 times more bits. When a receiver initially begins receiving a program on a particular channel, following a channel change or initial turning on of the receiver, it must wait until an I picture is received to begin decoding properly, which causes a delay.
To minimize channel change delay in digital video broadcast systems, I pictures are typically sent frequently, for example every N pictures. For example, to enable a ½ second delay (of the video compression portion of the system), it is common to use N=15 for 30 frames per second (fps) content. Since compressed I pictures are so much larger than compressed P and B pictures, this considerably increases the bitrate over what would be required if I pictures were not inserted so frequently.
In some systems, instead of sending full I pictures frequently, a technique called “progressive refresh” is used, where sections of pictures are intra coded. Typically, all macroblocks in the picture are intra-coded at least once during an N-picture period. In the JVT/H.264/MPEG AVC compression standard, P and B pictures may be predicted using multiple reference pictures, including the pictures before a preceding I picture. The standard identifies random access points as Independent Decoder Refreshes, or IDRs, which constrain that no reference pictures before the IDR are used in predicting pictures following the IDR. Pictures may be coded using slices of different types. A picture in which all coded slices are of type I may be referred to as an I picture.
The JVT/H.264/MPEG AVC compression standard includes a tool called redundant pictures, defined in the standard as follows:                redundant coded picture: A coded representation of a picture or a part of a picture. The content of a redundant coded picture shall not be used by the decoding process for a bitstream conforming to this Recommendation I International Standard. A redundant coded picture is not required to contain all macroblocks in the primary coded picture. Redundant coded pictures have no normative effect on the decoding process. See also primary coded picture.        
The slice header contains a redundant_pic_cnt field, whose semantics are defined in the JVT/H.264/MPEG AVC compression standard as follows:                redundant_pic_cnt shall be equal to 0 for slices and slice data partitions belonging to the primary coded picture. The redundant_pic_cnt shall be greater than 0 for coded slices and coded slice data partitions in redundant coded pictures. When redundant_pic_cnt is not present, its value shall be inferred to be equal to 0. The value of redundant_pic_cnt shall be in the range of 0 to 127, inclusive.                    If the syntax elements of a slice data partition A RBSP indicate the presence of any syntax elements of category 3 in the slice data for a slice, a slice data partition B RBSP shall be present having the same value of slice_id and redundant_pic_cnt as in the slice data partition A RBSP.            Otherwise (the syntax elements of a slice data partition A RBSP do not indicate the presence of any syntax elements of category 3 in the slice data for a slice), no slice data partition B RBSP shall be present having the same value of slice_id and redundant_pic_cnt as in the slice data partition A RBSP.                        
A system has been proposed wherein a channel change stream is encoded and transmitted along with the normal video bitstream. The channel change stream includes lower quality I pictures that are sent at a higher frequency than I pictures in the normal bitstream. When a user tunes to a new channel, playback could begin upon receipt of the first I pictures, in either the normal or channel change stream. This system is targeted at an end-to-end broadcast system, without any upstream indication of a channel change or possibility for storage at intermediate points in the system. By incorporating upstream channel change indications and/or intermediate storage points, the present invention can reduce bandwidth requirements over the most bandwidth critical links of the end-to-end system and increase channel change response time.
Another system has been proposed wherein a reduced resolution update codec is employed such that prediction residuals can be coded at lower resolutions for some of the coded pictures in a sequence, while other coded pictures in a sequence are coded at the full resolution. However, this system does not provide any capability for improved channel change efficiency.